A portion of a prior art flow meter 10 can be seen in FIG. 1, in which a pipe 12 is fitted with a wedge-shaped throttling element 14. The wedge element 14 has flat faces 16, 18 that are angled towards each other and terminate at a straight edge 20. The angle between the faces 16, 18 may be 90°, for example. Below the edge 20 there is a gap 22. Fluid flows into the pipe 12 at end 30, for example, passes through the gap 22 below the wedge 14, and out through the other end 32 of the pipe. The wedge 14 creates a pressure drop in the fluid within the pipe 12, which can be measured by calculating the difference between the upstream pressure measured at upstream port or pressure tap 34 and the downstream pressure measured at downstream port or pressure tap 36. The flow rate can then be calculated based on the pressure drop, properties of the fluid and the size (diameter) of the pipe.
Wedge-shaped throttling elements are used to measure the flow rates of viscous fluids and slurries or other solid-laden fluids, and are often used in the chemical industry. Compared to a throttling orifice, which is typically round with a diameter smaller than the pipe diameter, the wedge has better resistance to abrasion and a longer operating life.
In order to maintain accuracy of measurement, the edge 20 of the wedge 14 should not deteriorate with use. However, especially if the flow rates of abrasive fluids are being measured, then the edge 20 can wear quickly. This causes the gap 22 to open up after a short time of operation, changing the state of flow and worsening the accuracy and repeatability of measurement. Regarding the state of the flow, when the fluid passes through the gap 22, the cross-section of the flow rapidly decreases and the static pressure falls. Downstream of the wedge, pressure is recovered and a high speed swirling stream with bubbles may be generated. The resulting whirlpool washes against the edge 20 of the wedge 14 with an erosive effect, causing serious wear to the edge. Also, wear to the pipe wall just beyond the wedge occurs. Wear to the edge 20 eventually occurs even if the edge is hardened with stellite or tungsten carbide cladding. The accuracy of a wedge-type flow meter, if used for black water containing pulverized coal for example, may fall from about 0.5% to 5% within 2-3 months. In these cases, the wedge must be replaced regularly. In practice, it is not always convenient to replace the wedge, and in many cases the wedge is used until the meter itself is damaged or the accuracy has become extremely poor. The same problems apply to orifice throttles.
Venturi tubes are also used to measure flow rates, and consist of a gradually narrowing frustoconical section of pipe, followed by a short straight section and then a frustoconical section that opens up to the original diameter. While they generally have low head losses, they are only useful for very high Reynolds numbers, i.e. for fluids that have very low viscosity and that are clean, i.e. not laden with solids.
Referring to prior art FIG. 2, which shows a pipe 40 with an indented wall 42 that forms the wedge throttle, flow enters the pipe at end 44, in the direction of arrow 46, and then leaves at end 48. Flow of the fluid speeds up in the gap 50, causing turbulence 52 and erosion 54 of the wall of the pipe. As a consequence of the fluid impinging on the bottom of the pipe wall past the edge of the wedge 42, the fluid is reflected or directed upwards to the ceiling of the pipe wall to cause further erosion 56. Even if interior surfaces of the pipe 40 are coated with tungsten carbide at the locations of erosion, the coating wears off within several months of operation, and the pipe itself may become beyond repair in less than a year.
Furthermore, when wedge-type throttling elements are used for the flow of viscous fluids, the edge may become coked or fouled with dirt, which also reduces the accuracy of the measurement. Another problem with the wedge is that it has a relatively high head loss, corresponding to the unrecoverable energy loss from throttling back the fluid flow. Typically, the head loss of wedge-type meters is 30-50% of the measured differential pressure.
In order to ensure accurate measurements using the wedge-type meter, a straight section of pipe is needed upstream of the wedge, equal to at least 10× the diameter of the pipe. Likewise, a straight length that is at least 5× the pipe diameter is needed downstream of the wedge. These requirements may limit the applications in which the wedge meter may be installed.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.